Revisiting the venerable .38-55
- Thread starter Snakeoil
- Start date
Ian
Notorious member
My bad. I didn't read enough posts back. Who's on first!
CZ93X62
Official forum enigma
In keeping with my heretical mindset, I have pondered the practice of breech-seating and to some extent I understand the nuts and bolts of it.
I assume that breech-seating began during the black powder era. I have beem led to believe that airspace beween a column of black powder and a ball or bullet base was a recipe for chamber shattering. Can someone conversant on the practice fill in the wide gaps in my knowledge here? TIA.
I assume that breech-seating began during the black powder era. I have beem led to believe that airspace beween a column of black powder and a ball or bullet base was a recipe for chamber shattering. Can someone conversant on the practice fill in the wide gaps in my knowledge here? TIA.
Snakeoil
Well-Known Member
So, today's testing at Wilton was both fruitful and confusing. Let's start with fruitful.
I only loaded 20 rounds to conserve the 225 gr MBW bullets I have. But this morning when I walked into Charlie's on the bench was a Saeco 2-cavity mold for their 225 gr RNFPPB bullet. It is their 638 mold. Blocks look like brand new. I can now cast up a bunch with my 20:1 alloy and not have to be stingy with test rounds. That was fruitful piece #1.
At the range the objective was to see if going back to fixed ammo would reduce the effective case volume enough to reduce the vertical stringing at 500 yds that I experienced last Wed with the same bullet breech seated. In both today's and last week's testing, the load was the same 15 gr of 2400. The only change was going from breech seated bullets to fixed ammo. OAL for the rounds was 2.575 which had the first land just kissing the rifling. The wind was howling today so not a day for shooting groups. So, my focus was holding dead center on the Ram and looking for vertical consistency. The wind could do what it wanted with the windage placement. Fruitful piece #2 was once I was on the target and made no further sight or hold adjustments, just aimed at center. The rounds held vertically within 1MOA. I'm pretty happy with that. I forgot to take a pic of the target, but I stared at it long enough to remember the bullet placement. It will probably be untouched come Sunday morning and I can snap a pic when we are setting targets for the match. In the meantime, here is what I remember for shot placement.
The confusion comes in two separate pieces. First, my first shot out of the rifle, using a scope setting for the breech seated bullets put the round close to center and high in the back of the Ram. MV was 1505 fps. That's about 30 fps below the GRT prediction. The next round impacted in the dirt about 3 feet below the target. MV was 1416. Next one was in the dirt right next to it with an MV of 1406. I started cranking the scope up and eventually got on the target and then started shooting at center and just seeing where they went. I had another round do 1503 and impact high. I cannot explain those two rounds. After that, all the rounds were in the low 1400's and high 1300's. Avg MV was 1410 fps. I dropped the 2 1500 fps rounds from the group. I fired 20 rounds, deleted 2 so I should have 18 in the chrono. I only have 15. So, 3 more did not pick up. ES for those 15 was 74 fps and SD was 22 fps. Not horrible. But not great. The previous week's testing was 11 rounds with ES of 49 and SD of 16. So they were better statistically but strung vertically about 2 MOA.
The other confusion was GRT was way off in predicting MV. GRT predicted 1529 versus actual of 1410 fps. And, I would have expected the fixed ammo to produce higher MV than breech seated ammo and GRT predicted that, too. Yet the Avg MV for last week's testin was 1474 fps, which is 64 fps higher than today. Maybe the bullet jump allowance is inaccurate. I turned it off for the BS loads in GRT. I need to revist GRT and see if I did not make an input error.
So, next move is to bump up the powder charge. Looking to improve the ES and SD numbers and maintain accuracy at 500 yds. We have a centerfire match this coming Sunday. 100, 150, 200 and 300 yds with 2 offhand at 400. It's our Vermont Match. I just might bring the High Wall instead of the 03 and see how it does. Going to a match with only estimates for scope settings is not the best practice. But what the Hell. It's not like there is a Mercedes in the prize pool. We'll see what I want to do when I load on Saturday.
UPDATE: I just went into GRT and reset the seatin depth to zero (breech seated bullet) and turned off the bullet jump correction factor and it predicted 1478 fps for last week's loads. My chrono showed 1474 fps. That's pretty damn good. Cannot understand why my fixed ammo today was so much slower than predicted. Might be temperature. But ammo was in heated car for the ride up. Stayed in the car for maybe 1 hour and then we went to the club where the firing line is heated. I don't think that is it. Maybe my chrono got goofy today. But sure seemed to be fine.
UPDATE #2: Well, I had not changed the initial pressure estimate in GRT by inputting the caliber type, primer type and projectile type. This correction dropped the initial pressure from 3600 psi down to 1160. That then dropped the predicted MV to 1423 which is much closer to the 1410 I saw today. However, now when I go to the BS numbers, the prediction is what I would have expected, lower than for fixed ammo of the same load. So now I don't understan why the MV was so high last week. I think I need to stare at GRT a bit more. I must be making a rookie mistake because I'm new to this. It's all part of the learning experience.
I only loaded 20 rounds to conserve the 225 gr MBW bullets I have. But this morning when I walked into Charlie's on the bench was a Saeco 2-cavity mold for their 225 gr RNFPPB bullet. It is their 638 mold. Blocks look like brand new. I can now cast up a bunch with my 20:1 alloy and not have to be stingy with test rounds. That was fruitful piece #1.
At the range the objective was to see if going back to fixed ammo would reduce the effective case volume enough to reduce the vertical stringing at 500 yds that I experienced last Wed with the same bullet breech seated. In both today's and last week's testing, the load was the same 15 gr of 2400. The only change was going from breech seated bullets to fixed ammo. OAL for the rounds was 2.575 which had the first land just kissing the rifling. The wind was howling today so not a day for shooting groups. So, my focus was holding dead center on the Ram and looking for vertical consistency. The wind could do what it wanted with the windage placement. Fruitful piece #2 was once I was on the target and made no further sight or hold adjustments, just aimed at center. The rounds held vertically within 1MOA. I'm pretty happy with that. I forgot to take a pic of the target, but I stared at it long enough to remember the bullet placement. It will probably be untouched come Sunday morning and I can snap a pic when we are setting targets for the match. In the meantime, here is what I remember for shot placement.
The confusion comes in two separate pieces. First, my first shot out of the rifle, using a scope setting for the breech seated bullets put the round close to center and high in the back of the Ram. MV was 1505 fps. That's about 30 fps below the GRT prediction. The next round impacted in the dirt about 3 feet below the target. MV was 1416. Next one was in the dirt right next to it with an MV of 1406. I started cranking the scope up and eventually got on the target and then started shooting at center and just seeing where they went. I had another round do 1503 and impact high. I cannot explain those two rounds. After that, all the rounds were in the low 1400's and high 1300's. Avg MV was 1410 fps. I dropped the 2 1500 fps rounds from the group. I fired 20 rounds, deleted 2 so I should have 18 in the chrono. I only have 15. So, 3 more did not pick up. ES for those 15 was 74 fps and SD was 22 fps. Not horrible. But not great. The previous week's testing was 11 rounds with ES of 49 and SD of 16. So they were better statistically but strung vertically about 2 MOA.
The other confusion was GRT was way off in predicting MV. GRT predicted 1529 versus actual of 1410 fps. And, I would have expected the fixed ammo to produce higher MV than breech seated ammo and GRT predicted that, too. Yet the Avg MV for last week's testin was 1474 fps, which is 64 fps higher than today. Maybe the bullet jump allowance is inaccurate. I turned it off for the BS loads in GRT. I need to revist GRT and see if I did not make an input error.
So, next move is to bump up the powder charge. Looking to improve the ES and SD numbers and maintain accuracy at 500 yds. We have a centerfire match this coming Sunday. 100, 150, 200 and 300 yds with 2 offhand at 400. It's our Vermont Match. I just might bring the High Wall instead of the 03 and see how it does. Going to a match with only estimates for scope settings is not the best practice. But what the Hell. It's not like there is a Mercedes in the prize pool. We'll see what I want to do when I load on Saturday.
UPDATE: I just went into GRT and reset the seatin depth to zero (breech seated bullet) and turned off the bullet jump correction factor and it predicted 1478 fps for last week's loads. My chrono showed 1474 fps. That's pretty damn good. Cannot understand why my fixed ammo today was so much slower than predicted. Might be temperature. But ammo was in heated car for the ride up. Stayed in the car for maybe 1 hour and then we went to the club where the firing line is heated. I don't think that is it. Maybe my chrono got goofy today. But sure seemed to be fine.
UPDATE #2: Well, I had not changed the initial pressure estimate in GRT by inputting the caliber type, primer type and projectile type. This correction dropped the initial pressure from 3600 psi down to 1160. That then dropped the predicted MV to 1423 which is much closer to the 1410 I saw today. However, now when I go to the BS numbers, the prediction is what I would have expected, lower than for fixed ammo of the same load. So now I don't understan why the MV was so high last week. I think I need to stare at GRT a bit more. I must be making a rookie mistake because I'm new to this. It's all part of the learning experience.
Last edited:
Bret4207
At the casting bench in the sky. RIP Bret.
You need to Get Ned Roberts, "The Breech Loading Single shot Rifle". Good stuff.
Snakeoil
Well-Known Member
Charlie probably has it. I'll borrow it after I finish Vol II of Winchester Single Shots. Have not been doing much reading lately.
Snakeoil
Well-Known Member
So, with a good night's sleep and a dreary day to not make me feel guilty about staying indoors, I revisited GRT and the chrono results from my last few weeks of testing. I realized I was making the common mistake of assuming one parameter was in play when in reality, we know that is hardly ever the case.
I brougnt up the two breech seated loads, one with a 277 gr bullet over 17 gr of 2400 and the other a 225 gr bullet over 15 gr of 2400 and looked at input data that might effect the difference between GRT predicted MV and the chrono readings. Having two different BS examples helped confirm assumptions. My thought was if the assumption applied to one load had the same effect on the other load, then there is a good chance that the assumption is legit. Then I moved to the chrono and GRT data from yesterday's fixed ammo test and tried to see if the same assumptions might apply.
So here is what I'm talking about. I was thinking that BS rounds created a larger effective case volume and this was throwing off the MV prediction. But in reality, since the total case volume is known and I'm seating with zero depth, the total case volume is known and applies. So, there is no error created by breach seating when it comes to effective case volume. So, what else could be affecting the predicted versus actual MV numbers?
GRT has some input cells I will call "correction factors". These are factors that are involved and must be considered. But they are pretty much impossible for a reloader to know or calculate. One is Initial Pressure. This is the pressure when the bullet first starts to move. It is based upon primer energy, friction between the bullet and the case (crimp, neck tension, seating depth) and press-in force into the rifling. Let's compare fixed ammo to breech seated ammo for this parameter.
Fixed Ammo - No crimp, 0.002 neck tension
So when the primer fires, the bullet starts to move in the case. This increases effective volume. The bullet builds inertia and hits the rifling. The inertia helps carry it into the rifling as the pressure continues to build until it reaches its peak chamber pressure.
Breech Seated Ammo - No crimp, no neck tension, already engraved into the rifling
Primer fires and pressure starts to build. Effective case volume is greater since bullet is not taking space in the case. But the bullet is not moving. Pressure builds against the stationary bullet. Since the engraved bullet has to overcome static friction of the rifling the pressure builds higher until the bullet starts to move.
Now, I believe that the pressure required to push the bullet out of the case is less than the pressure required to push the bullet thru the rifling. I tend to believe that the primer has no effect on the BS bullet moving as it does when in fixed ammo. I also suspect that the pressure builds slower in the fixed ammo because the bullet starts to move with the primer, making more space. And since it is moving, there is no static friction to overcome when it hits the rifling. Plus a body in motion tends to stay in motion. So as the powder burns and the pressure builds, the bullet keeps moving and when it hits the rifling that inertia helps it enter and continue down the barrel. So the peak pressure should be lower and occurs at a later point in time.
In the BS bullet, the primer fires and the powder starts to burn and pressure starts to rise. The bullet is stationary and must overcome static friction. Plus a body at rest tends to stay at rest. Hence the bullet stays put. So the pressure builds and builds faster and higher until the bullet starts to move. This tells me that the pressure will build faster and higher as there is no relief until the bullet starts to move.
It was this thought process that made me look hard at initial pressure. One would think that because the BS bullet has created more effective case volume, that initial pressure would be lower than for fixed ammo. But I believe that the static friction of the BS bullet causes the pressure to rise faster and higher, similar to a plugged barrel. So, the analogy I would use is the fixed ammo shoves the bullet and the BS ammo punches the bullet. In order to account for this, I think that the initial pressure for the BS bullet should be higher. GRT asks 3 quetions to determine initial pressure.
I used "standard to brittle" for the lead material for BS and "soft lead" for fixed. I did that to account for static friction of the BS bullet in the rifling. And damn if the MV numbers did not suddenly fall in line (or be pretty close) to what I saw over the chronograph. The result was about a 1000 psi difference in initial pressure between the two loads.
Anyone reading this with a good ballistics background or similar, please comment. If I am right, this will help me "accurize" GRT to cast bullet shooting by varying the intial pressure when comparing BS to fixed ammo.
Here are the GRT numbers for each. Note that the BS ammo reached peak pressure faster and his higher than the fixed. Same bullet, same powder charge.
I brougnt up the two breech seated loads, one with a 277 gr bullet over 17 gr of 2400 and the other a 225 gr bullet over 15 gr of 2400 and looked at input data that might effect the difference between GRT predicted MV and the chrono readings. Having two different BS examples helped confirm assumptions. My thought was if the assumption applied to one load had the same effect on the other load, then there is a good chance that the assumption is legit. Then I moved to the chrono and GRT data from yesterday's fixed ammo test and tried to see if the same assumptions might apply.
So here is what I'm talking about. I was thinking that BS rounds created a larger effective case volume and this was throwing off the MV prediction. But in reality, since the total case volume is known and I'm seating with zero depth, the total case volume is known and applies. So, there is no error created by breach seating when it comes to effective case volume. So, what else could be affecting the predicted versus actual MV numbers?
GRT has some input cells I will call "correction factors". These are factors that are involved and must be considered. But they are pretty much impossible for a reloader to know or calculate. One is Initial Pressure. This is the pressure when the bullet first starts to move. It is based upon primer energy, friction between the bullet and the case (crimp, neck tension, seating depth) and press-in force into the rifling. Let's compare fixed ammo to breech seated ammo for this parameter.
Fixed Ammo - No crimp, 0.002 neck tension
So when the primer fires, the bullet starts to move in the case. This increases effective volume. The bullet builds inertia and hits the rifling. The inertia helps carry it into the rifling as the pressure continues to build until it reaches its peak chamber pressure.
Breech Seated Ammo - No crimp, no neck tension, already engraved into the rifling
Primer fires and pressure starts to build. Effective case volume is greater since bullet is not taking space in the case. But the bullet is not moving. Pressure builds against the stationary bullet. Since the engraved bullet has to overcome static friction of the rifling the pressure builds higher until the bullet starts to move.
Now, I believe that the pressure required to push the bullet out of the case is less than the pressure required to push the bullet thru the rifling. I tend to believe that the primer has no effect on the BS bullet moving as it does when in fixed ammo. I also suspect that the pressure builds slower in the fixed ammo because the bullet starts to move with the primer, making more space. And since it is moving, there is no static friction to overcome when it hits the rifling. Plus a body in motion tends to stay in motion. So as the powder burns and the pressure builds, the bullet keeps moving and when it hits the rifling that inertia helps it enter and continue down the barrel. So the peak pressure should be lower and occurs at a later point in time.
In the BS bullet, the primer fires and the powder starts to burn and pressure starts to rise. The bullet is stationary and must overcome static friction. Plus a body at rest tends to stay at rest. Hence the bullet stays put. So the pressure builds and builds faster and higher until the bullet starts to move. This tells me that the pressure will build faster and higher as there is no relief until the bullet starts to move.
It was this thought process that made me look hard at initial pressure. One would think that because the BS bullet has created more effective case volume, that initial pressure would be lower than for fixed ammo. But I believe that the static friction of the BS bullet causes the pressure to rise faster and higher, similar to a plugged barrel. So, the analogy I would use is the fixed ammo shoves the bullet and the BS ammo punches the bullet. In order to account for this, I think that the initial pressure for the BS bullet should be higher. GRT asks 3 quetions to determine initial pressure.
- Caliber type- rifle, pistol, or shotgun
- Projectile type - material type soft lead, std to brittle lead, jacketed, solid copper, etc.
- Primer type - standard or magnum.
I used "standard to brittle" for the lead material for BS and "soft lead" for fixed. I did that to account for static friction of the BS bullet in the rifling. And damn if the MV numbers did not suddenly fall in line (or be pretty close) to what I saw over the chronograph. The result was about a 1000 psi difference in initial pressure between the two loads.
Anyone reading this with a good ballistics background or similar, please comment. If I am right, this will help me "accurize" GRT to cast bullet shooting by varying the intial pressure when comparing BS to fixed ammo.
Here are the GRT numbers for each. Note that the BS ammo reached peak pressure faster and his higher than the fixed. Same bullet, same powder charge.
Last edited:
fiver
Well-Known Member
there's a couple of things.
i'm kind of too tired to go into them right now, but your real close on the majority of your assumptions,ideas-hypothesis... as in educated guess.
so it won't be until tomorrow night, unless Ian weighs in with some of the stuff we have discussed before..
i'm kind of too tired to go into them right now, but your real close on the majority of your assumptions,ideas-hypothesis... as in educated guess.
so it won't be until tomorrow night, unless Ian weighs in with some of the stuff we have discussed before..
Bret4207
At the casting bench in the sky. RIP Bret.
Not you, Allen, CZ93x62.
fiver
Well-Known Member
let's start here.
The bullet builds inertia and hits the rifling.
at this point in time there will be a little pressure spike [which GRT doesn't show] then a short fast drop before the climb to the top.
the pressure curve will look like your chart shows only with a quick blip about 1/4-1/3 of the way up.
this spike is also present in breech fixed ammo, only it is manipulated into the pressure rise and adds more pressure from/to the start.
now the reason your needing more 2400 to get the same/same is because your turning the 45-70 into a 45-80, you need the extra powder to make more gas to fill up the extra volume this then will break the friction and move the bullet and pressure will level off slightly before quickly starting to falling off,,,, as soon as the bullet moves.
nuthin new the graph is showing exactly that happening in real time.
AIRC that other predictor program will calculate the pressure rise from a stopped in place bullet.
in my XCB with the bullets jammed into the rifling and then the case slid over the back with about .001 neck tension we figured a pressure rise before movement of around 3500-4K.
this was done on purpose [by me] to help insure the powder was all consumed, that the bullet was moved as little as possible to enter the barrel fully, and to get the manipulated case as straight as possible into the chamber.
Bret4207
At the casting bench in the sky. RIP Bret.
I like it when you get thinking out loud and I suddenly realize I missed a part of the process for 40 years!
Snakeoil
Well-Known Member
First, my goal is for this to be a discussion, not an argument. I'm just saying this up front because I may challenge some of your thoughts. Not trying to be the newbie know-it-all. But it is just my nature to challenge points that I either do not agree with or that the data does not support. But I realize I might be missing something so I'm not afraid to be proven wrong. So, see my comments in your reply below. And thanks for taking the time to discuss this.
Second, above I posted two charts for fixed ammo using the different initial pressures by mistake. I just noticed this when I went to refer to them as I typed. One should have been for BS ammo with a different initial pressure. That's what I get for doing stuff at the end of the day. My apologies for that. I was looking at the correct results as I typed my original post. So, I am going to edit the above post by deleting one of the charts and replacing it with the correct chart. But I won't be able to do that until after I complete this post. So, please take another look at them.
My comments are in bold in your responses below.
Second, above I posted two charts for fixed ammo using the different initial pressures by mistake. I just noticed this when I went to refer to them as I typed. One should have been for BS ammo with a different initial pressure. That's what I get for doing stuff at the end of the day. My apologies for that. I was looking at the correct results as I typed my original post. So, I am going to edit the above post by deleting one of the charts and replacing it with the correct chart. But I won't be able to do that until after I complete this post. So, please take another look at them.
My comments are in bold in your responses below.
Last edited:
fiver
Well-Known Member
i'm fine with any back and forth you wanna do.
i've been trying to get a good deeper discussion going here for a while.
i could use a little correction on stuff i mis-remember from time to time, and as long as it helps everyone else understand the process along the way i'm golden.
i've been trying to get a good deeper discussion going here for a while.
i could use a little correction on stuff i mis-remember from time to time, and as long as it helps everyone else understand the process along the way i'm golden.
fiver
Well-Known Member
okay lets discuss what i wrote in the quote below the last one you responded too.
re-look at what i said there about the initial pressure before the bullet moved.
your bottom half of the reply is correct.
the faster powder has a sooner spike but a shorter life as the powder is consumed quicker.
one of the things you have to take into consideration with the jammed bullet is that the primer does not have enough oomph to move it any further....
but it does send a sinewave of pressure that deflects off the base of the bullet, or off the base of a filler and then the bullet.
that little bit of initial pressure can move a filler forward if it isn't jammed up against the bullet base.
that's a little off topic for now, but it does need to be mentioned.
stuff like the 45-70 is a big open volume of space it doesn't work with the slower [say 4064 on back] powders because they won't burn fast enough too keep on filling the volume created by the bullet moving, as soon as the bullet moves ahead 1/2" you just added in about 30% more case/bore volume.
something like 2400 will because you've burned most of it and filled out the area with almost all of the gas your gonna make, now if the bullet gets a run at the rifling scoots in and goes you've built up the pressure to push it.
the further distance means more gas to first fill the area and then shove the bullet you end up walking a pretty fine line between the powder burning off and keeping the extra space full and it just not being able to do it.
switch to a slower more volume of powder and gas and you gain back the speed you lost waffling back and forth at the pressure/volume window at the end of the sequence.
it has consumed more powder at a slower speed to get to the same pressure needed to move that jammed bullet.
from that point on it may or may not be able to keep up too.
simply using more of the faster powder gives you more pressure too soon.
anyway.
part of what your seeing is the effect of a square case.
stuff like an Ackley improved round will show a powder to burn one step faster too.
as in one more grain of powder than the parent case is using will quite often show another 100 fps. of velocity increase even though it is in a 10% larger volume.
this stuff is weird.
and it also explains why many guys will do load development by moving the burn speed of the powder up/down a notch to try and tune things in rather than chase the bullet jump for the first step.
re-look at what i said there about the initial pressure before the bullet moved.
your bottom half of the reply is correct.
the faster powder has a sooner spike but a shorter life as the powder is consumed quicker.
one of the things you have to take into consideration with the jammed bullet is that the primer does not have enough oomph to move it any further....
but it does send a sinewave of pressure that deflects off the base of the bullet, or off the base of a filler and then the bullet.
that little bit of initial pressure can move a filler forward if it isn't jammed up against the bullet base.
that's a little off topic for now, but it does need to be mentioned.
stuff like the 45-70 is a big open volume of space it doesn't work with the slower [say 4064 on back] powders because they won't burn fast enough too keep on filling the volume created by the bullet moving, as soon as the bullet moves ahead 1/2" you just added in about 30% more case/bore volume.
something like 2400 will because you've burned most of it and filled out the area with almost all of the gas your gonna make, now if the bullet gets a run at the rifling scoots in and goes you've built up the pressure to push it.
the further distance means more gas to first fill the area and then shove the bullet you end up walking a pretty fine line between the powder burning off and keeping the extra space full and it just not being able to do it.
switch to a slower more volume of powder and gas and you gain back the speed you lost waffling back and forth at the pressure/volume window at the end of the sequence.
it has consumed more powder at a slower speed to get to the same pressure needed to move that jammed bullet.
from that point on it may or may not be able to keep up too.
simply using more of the faster powder gives you more pressure too soon.
anyway.
part of what your seeing is the effect of a square case.
stuff like an Ackley improved round will show a powder to burn one step faster too.
as in one more grain of powder than the parent case is using will quite often show another 100 fps. of velocity increase even though it is in a 10% larger volume.
this stuff is weird.
and it also explains why many guys will do load development by moving the burn speed of the powder up/down a notch to try and tune things in rather than chase the bullet jump for the first step.
Snakeoil
Well-Known Member
Well, just to keep this on the same page, I'm not comparing powders with different burn rates. All this is being done with 2400. And the caliber is .38-55, not .40-65 or .45-70.
The reflected shock wave from the primer is something I had not considered. And this could be a contributor to why I had to bump up the initial pressure with the BS load to see the predicted MV match the chrono readings I was seeing.
If you look back at the curves, you'll see that you never run out of powder burn with 2400. Both fixed and BS shows powder burns in the mid-80% range. So, you are making gas all the way down the barrel. And since the bullet velocit curve does not flatten out, you are making enough for it to continue to accelerate down the barrel, all the way to the muzzle.
But I have to go back to your returning shock wave. I think that, combined with the plugged chamber with a BS bullet is why the initial pressure must be higher than with fixed ammo. Another hint to the intial pressure being higher is look at the very start of the bullet velocity curve. The BS bullet is a steep straight line where the fixed bullet kinda crawls from the start before it starts to run. That falls in line with the static friction element that must be overcome. More pressure to start means once it breaks free it takes off like a rabbit.
I'm going to load up some more test rounds for this Wed. Plan is to load 17gr of 2400. Intent is to up the pressure a bit to lower the ES and SD numbers or at least see if they respond like I expect them to. Back last fall when I was doing fixed ammo testing, the best SD numbers were with 15.5 of 2400 behind a 250 gr bullet. So, I'm going to try and duplicate the pressure with a 225 gr bullet by upping the charge. I realized I'll exceed the MV of the 250 gr bullet. But that's okay if the accuracy holds or improves and the ES and SD improve. So, I'll use GRT to predict the right charges to get similar pressures with the lighter bullet.
As a side note, at the match today, a member shooting a CPA 44-1/2 in .32-40 thinks he doubled charged a case. The primer was about as flat as it could possibly be. You could barely see the seam between the primer and the base of the case. I asked what his load was and when I got home I ran it in GRT with his bullet and his OAL. GRT says he was just a whisker short of 70Kpsi. That CPA is a lot stronger than I thought. He's a very lucky boy. That could have been very ugly.
The reflected shock wave from the primer is something I had not considered. And this could be a contributor to why I had to bump up the initial pressure with the BS load to see the predicted MV match the chrono readings I was seeing.
If you look back at the curves, you'll see that you never run out of powder burn with 2400. Both fixed and BS shows powder burns in the mid-80% range. So, you are making gas all the way down the barrel. And since the bullet velocit curve does not flatten out, you are making enough for it to continue to accelerate down the barrel, all the way to the muzzle.
But I have to go back to your returning shock wave. I think that, combined with the plugged chamber with a BS bullet is why the initial pressure must be higher than with fixed ammo. Another hint to the intial pressure being higher is look at the very start of the bullet velocity curve. The BS bullet is a steep straight line where the fixed bullet kinda crawls from the start before it starts to run. That falls in line with the static friction element that must be overcome. More pressure to start means once it breaks free it takes off like a rabbit.
I'm going to load up some more test rounds for this Wed. Plan is to load 17gr of 2400. Intent is to up the pressure a bit to lower the ES and SD numbers or at least see if they respond like I expect them to. Back last fall when I was doing fixed ammo testing, the best SD numbers were with 15.5 of 2400 behind a 250 gr bullet. So, I'm going to try and duplicate the pressure with a 225 gr bullet by upping the charge. I realized I'll exceed the MV of the 250 gr bullet. But that's okay if the accuracy holds or improves and the ES and SD improve. So, I'll use GRT to predict the right charges to get similar pressures with the lighter bullet.
As a side note, at the match today, a member shooting a CPA 44-1/2 in .32-40 thinks he doubled charged a case. The primer was about as flat as it could possibly be. You could barely see the seam between the primer and the base of the case. I asked what his load was and when I got home I ran it in GRT with his bullet and his OAL. GRT says he was just a whisker short of 70Kpsi. That CPA is a lot stronger than I thought. He's a very lucky boy. That could have been very ugly.